This invention relates generally to optical display systems and, more particularly, has reference to a new and improved system for displaying instruments in an automobile.
General production line automobiles typically have a plurality of instruments, indicators and gauges displayed on a dashboard panel behind the steering wheel. These instruments usually include a speedometer, a tachometer, a clock, an odometer, and a trip odometer, various auxiliary gauges for oil pressure, engine temperature, fuel level and battery charge, and a collection of system warning lights. In older cars, the instruments are often electro-mechanical devices with moving needle indicators. Newer models frequently use backlit direct view liquid crystal displays or self-illuminating vacuum flourescent displays.
Due to the limited space available in the interior of an automobile, the instrument panel is usually located relatively close (e.g. about two feet) to the driver's eyes. To read direct view instruments, the driver refocuses his eyes from the far range viewing (essentially at infinity) used to observe the road ahead to the near range viewing used to look at the instruments. While such systems generally have served their purpose, there remains a continuing desire for further improvements, particularly in the areas of instrument readability and reduced driver eye strain.
One foreign automobile manufacturer attempted to provide an improved instrument display with a viewing distance slightly beyond the normal dashboard panel position by mounting a flat folding mirror on the dashboard below a full-size vacuum fluorescent instrument display. This approach was unsatisfactory in several respects and left considerable room for improvement.
A need exists for an automobile instrument display system which minimizes driver eye strain and enhances instrument readability, particularly for older persons and persons who are far sighted or wear bifocals, by producing an instrument cluster image well beyond (e.g., about one foot or more) the face of the dashboard and a considerable distance (e.g., about four feet or more) from the driver's normal viewing position. The desired system would be configured to fit within the existing space/volume currently occupied by the conventional dashboard instrument panel, would provide a display format and viewing angle conditions which were similar to conventional direct view instrument clusters, would provide a display image having comfortable visibility and legibility under all ambient light conditions, would be mass-producible at a cost comparable to a conventional direct view instrument cluster, would be simple in structure, would have an electrical interface which was compatible with an automotive electrical system, and would provide good optical characteristics, especially as regards to image quality, disparity and color. Numerous problems are encountered in attempting to satisfy those needs.
For example, optical complications are caused by geometric conditions which are encountered in the typical automobile environment. For instance, the driver's head and eyes normally do not remain stationary but move throughout an elliptical viewing area known as the eye motion box or the eyellipse. Driver's also have different seated body lengths and prefer different seat height and position adjustments. An eyellipse of about 8" H .times.5" V .times.10" D centered at about 30.5" from the instrument panel will accommodate most of the driver population. The typical instrument panel viewing angle (i.e., the line-of-sight used to see the instrument panel from the eyellipse) is about 19.degree. below horizontal and the angular subtense (i.e., the amount of scan used to see the entire instrument display) is about 24.degree. H .times.6.degree. V.
Additional complications are caused by the problem of vertical disparity or dipvergence. When an object field is viewed through an optical system, each eye typically sees a somewhat different view. Vertical disparity is the angular difference along the vertical axis of an object point as viewed by each eye. Vertical disparity has a bearing upon driver viewing comfort. A driver's tolerance limit to vertical disparity influences the complexity of the display optics. An instrument display system should reduce vertical disparity to a level which is commensurate with driver comfort while not unduly complicating the display optics.
Still further complications are caused by the high ambient light conditions which are present in most automobiles. Ambient light includes direct sunlight and specular reflections from surrounding objects which can shine into the driver's eyes and reduce display visibility. The instantaneous dynamic range of an eye adapted to a typical horizon sky luminance of about 3,000 foot-Lamberts (fL) is on the order of about 600:1. Hence, the black level for this eye is about 5 fL and all stimuli at luminance levels of 5 fL or less look equally black. Hence, even if there were no transmission losses and no noise (i.e., ambient light falling on and being reflected from the display), the luminance desired for the bright symbols of an instrument display in order to provide the 2:1 contrast generally regarded as adequate for viewing line/graphic images would be about 10 fL. This brightness should be provided by the electrical power available in an automobile.
A uniform high contrast and uniform bright image of the instrument is also desired, even in these high ambient light conditions. However, the two conventional ways to diffuse light across a viewing area, i.e., opaque lambertian diffusion and high gain backlit diffusion, may be unsatisfactory in certain situations. In the case of lambertian diffusion, the light is scattered equally in all directions. Where the optical system utilizes the diffused light only within a small angular cone directed into the eye motion box, radiation outside this cone tends to become stray light which causes high background levels and reduced contrast ratio. High gain backlit diffusing screens scatter the light into a narrower angular cone and thus improve the effective optical efficiency, but the resulting display uniformity over the viewing area can be unsatisfactory. There may be an undesirable drop-off in brightness at the edge of the eye motion box.
Additional complications arise from the desire to provide a display system which is harmonious with the general styling of the automobile interior and which has high customer acceptance and appeal. In this regard, the system should be packaged to fit within the existing dashboard space now occupied by a conventional direct view instrument display, should be mass-producible at a reasonable cost, should provide a multi-color image, and should provide an image source whose stability, drift, latency and persistence are such that the image is not difficult to interpret nor aesthetically objectionable.
The present invention overcomes these problems and satisfies the need for an improved instrument display system.